1. Field of the Invention
The present invention relates to a hydrostatic bearing apparatus for rotatably supporting a spindle by means of hydrostatic pressure generated in fluid pockets formed at an internal surface of a bearing bushing.
2. Description of the Prior Art
In a known hydrostatic bearing, pressurized fluid is supplied via respective throttle elements to fluid pockets which are circumferentially formed at an internal surface of a bearing bushing, and a spindle received in the bearing bushing is rotatably carried by means of hydrostatic pressure generated in each of the fluid pockets. The rotation of the spindle causes pressurized fluid in each fluid pocket to circumferentially flow as a result of being involved by the outer circumferential surface of the spindle. Thus, heated pressurized fluid in each fluid pocket flows therefrom into another fluid pocket next thereto at the downstream side in the rotational direction of the spindle. This phenomenon is called "carry-over". The "carry-over" phenomenon takes place not only in a hydrostatic bearing which has no exhaust groove between every two circumferentially successive fluid pockets, but also in another hydrostatic bearing which has an exhaust groove between every two circumferentially successive fluid pockets.
That is, the exhaust groove is filled with heated fluid which overflows a fluid pocket located at the upstream side in the spindle rotational direction. The rotation of the spindle thus causes the heated fluid to flow from each fluid pocket to another fluid pocket next thereto at the downstream side of the spindle rotational direction. The pocket-to-pocket flow of the heated fluid is repeated, whereby the generation of heat is facilitated.
Generally, the fluid pockets are supplied with pressurized fluid through respective throttle elements. Pressure loss caused by the throttle elements is converted into thermal energy. In addition, rotation of the spindle is accompanied by fluid friction, which results in heat generation. In an ideal state wherein no "carry-over" phenomenon takes place, the temperature of the bearing hardly increases since heated pressurized fluid is completely discharged from the fluid pockets outside the bearing and after cooled off by a cooler device, is supplied again from a fluid pump to the fluid pockets. In a state wherein the "carry-over" phenomenon takes place, on the other hand, pressurized fluid once heated by experiencing the "carry-over" flow is again heated in another fluid pocket to expedite an temperature increase at the fluid pocket even where thermally controlled fresh fluid is supplied to the fluid pockets.
Such temperature increase causes thermal expansion of the spindle, the bearing bushing and the bearing housing and leads to machining error caused by thermal deformation of these machine tool components. Such machining errors resulting from heat generation still remain as a major problem to be solved in a hydrostatic bearing for precision machine tools.
To solve these drawbacks, a known fluid bearing is provided in a bearing housing thereof with a water jacket designed to diminish the temperature increase of a bearing bushing thereof. However, the provision of the water jacket is not effective to prevent heat generation at the internal surface of the bearing bushing and raises a difficulty in designing down-sized hydrostatic bearings.